Study On Corrosion-induced Interactions Between Silicon Carbide And Nickel-based Alloy In Molten FLiNaK Salt

Molten salt reactor（MSR）,one of six reactors proposed by the Generation IV International Forum,exhibits unique advantages,such as inherent safety,water-free cooling,and small modularization.MSRs employ nickel-based alloys as main structural materials,molten fluoride salts as coolant and fuel carrier.Silicon carbide（Si C）and its composites have been proposed to be used as shutdown rod channels and coatings for fuel particles in the MSRs because of their outstanding high-temperature properties,low neutron absorption,and low tritium permeation/retention.As molten fluoride salts are highly corrosive,the corrosion resistance of Si C materials in molten fluoride salts is one of the key bases for evaluating the usability of Si C in MSRs.It had been studied that Si C-based materials exhibite resistance to intrinsic corrosion induced by alkali fluorides.However,during the operation of MSRs,nickel-based alloys can be corroded by impurities in molten fluoride salts,which causes that the molten fluoride salt contain a small amount of metallic corrosion products that may affect the corrosion of Si C.Furthermore,the corrosion products of Si C in molten fluoride salt may also affect the corrosion of metallic materials.Therefore,in view of the corrosion of Si C materials in MSRs,the effect of nickel-based alloy on the corrosion of Si C in a molten FLi Na K salt was studied.The effect of Cr³⁺ ,one of main corrosion products from nickel-based alloys,on the corrosion of Si C and the related mechanism were studied.The interactions between Si C materials and nickel-based alloys in the molten FLi Na K salt were further studied.Main conclusions of this paper are summarized as follows:The static corrosion experiments were carried out in a silicon carbide crucible with nickel-based alloy and Si C materials.Results corroborate the interactions between nickel-based alloy and silicon carbide in molten FLi Na K salt,which results in the formation of nickel silicide on the surface of nickel-based alloy and chromium carbide on the surface of Si C.The corrosion of nickel-based alloys in the molten FLi Na K salt is mainly related to the diffusion and dissolution of Cr,and Cr in the alloy is selectively corroded and forms chromium fluoride into the molten fluoride salt.The effect of Cr³⁺ on the corrosion of CVD Si C was performed by static corrosion experiments in a purified molten FLi Na K salt and some CrF_3-doped FLi Na K molte salt,respectively.Results show that the purified FLi Na K salt slightly corrode CVD Si C,which is mainly attributed to uniform corrosion driven by oxygen impurities in the salt and Si C materials.The corrosion results in the dissolution of Si in Si C materials into the molten salt and weight loss of the CVD Si C sample after corrosion.In the CrF_3-doped FLi Na K molte salts,with the increase of initial Cr³⁺ concentration in the salts,the corrosion behavior of Si C gradually changes from weight loss to weight gain,and Si content in the molten salt increases after corrosion.The weight gain of Si C is mainly due to the reaction of Cr³⁺ with Si C,forming chromium carbide（Cr₇C₃,Cr₃C₂）on the surface of Si C,which is consistent with the corrosion results of nickel-based alloy and silicon carbide in the same crucibles.Therefore,the chromium fluoride formed by corrosion of nickel-based alloys is one of the effects of nickel-based alloys on the corrosion of Si C materials in the molten FLi Na K salts.Interactions between Si C and metallic materials including nickel-based alloy（GH3535）,316 L stainless steel,Ni and Fe metal in the FLi Na K molten salt were investigated.Results show that,in the case that Si C and nickel-based alloy coexist in the molten FLi Na K salt,corrosion products from one material will further react with the other and finally aggravate the corrosion of both materials.At the initial stage of corrosion,the molten salt,together with its initial impurities(Fe^2+/3+,Ni²⁺ and moisture,for example),react with Cr of the alloy to form chromium fluorides,which results in the dissolution of chromium from alloy into molten salt.Similarly,the corrosion of Si C causes the dissolution of Si from Si C into the molten salt.Ni₃₁Si₁₂,Ni₃ Si,Cr Fe₈ Si,and Cr₃Si are formed on the surface of GH3535 alloy,Ni metal,316 L SS,and Cr metal,respectively,which indicates that the corrosion products of Si C in the molten FLi Na K salt react with Ni,Cr,and Fe.Comparing the corrosion results of GH3535 alloy in two corrosion systems with or without Si C,it is found that the corrosion products of Si C in the molten FLi Na K salt accelerate the corrosion of alloys,which enhances the depletion of chromium in GH3535 alloy and promote the diffusion of Ni to the outer surface of alloy,and increases the depth of intergranular corrosion of GH3535 alloy.Meanwhile,chromium fluorides,main corrosion products of GH3535 alloy dissolved in the molten salt,can further react with Si C materials to from chromium carbide and silicon fluoride,which promotes the dissolution of Si from Si C into the molten FLi Na K salt.